This invention relates to the art of electrostatographic copying and more specifically to a method of preparing stable dispersions of selenium which can be cast into thin films. The art of electrostatographic copying, originally disclosed by C. F. Carlson in U.S. Pat. No. 2,297,691, involves as an initial step, the uniform charging of a plate comprised of a conductive substrate normally bearing on its surface a non-conductive barrier layer which is covered by a layer of a photoconductive insulating material. This is followed by exposing the plate to activating radiation in imagewise configuration which results in dissipation of the electrostatic charge in a pattern known as the latent image. The latent image is developed by contacting it with an electroscopic marking material commonly referred to as toner. This material is electrostatically attracted to the latent image which is, by definition, in the configuration of those portions of the photoreceptor which were not exposed to the activating radiation. The toner image may be subsequently transferred to paper and fused to it to form a permanent copy. Following this, the latent image is erased by discharging the plate and excess toner is cleaned from it to prepare the plate for the next cycle.
A photoconductive material which has achieved considerable commercial success is amorphous selenium. This material is applied to the conductive substrate by vapor deposition techniques which provide the desired photosensitive plate. Trigonal selenium in a binder matrix may also be used in some cases. A less expensive way to apply a layer of selenium would be to cast the layer from a colloidal dispersion of selenium. Such a process is problematical, however, because of the difficulties encountered in forming stable colloidal dispersions of selenium at reasonable selenium concentrations.
Colloidal dispersions of selenium have been prepared by reacting hydrazine with selenous acid in an aqueous solution containing gelatin. The preparation of monodisperse selenium sols formed on gold nucleation sites is reported in the Journal of Colloidal and Interface Science, Symposium Issue, pp. 227-235, June 1968. Those dispersions are, however, of very low concentration which renders their use in practical applications problematical.
It is known that hydrazine will react with selenous acid to form zero valent selenium by the reaction: EQU N.sub.2 H.sub.4 + H.sub.2 SeO.sub.3 .fwdarw.Se.degree. + 3H.sub.2 O + N.sub.2
this reaction, when carried out in aqueous solution, does not normally provide stable, colloidal dispersions of selenium due to the tendency of the selenium to flocculate out of the medium.
It would be desirable and it is an object of the present invention to provide a novel process for the formation of stable, colloidal dispersions of selenium.
Another object is to provide such a process in which colloidal concentrations containing up to 25 weight percent selenium and greater can be formed.
A further object is to provide such a process in which the selenium is formed by the reduction of selenous acid or an alkyl selenite by hydrazine or a substituted hydrazine.